1. Field of the Invention
The present invention relates to an infrared ray detector of dual structure employing infrared ray detecting elements for detecting an intruder or the like.
2. Description of the Prior Art
In recent years, a pyroelectric infrared ray sensor has generally been employed as a detector for an intruder or the like since the same is easy to set and handle in comparison with an LED. The pyroelectric infrared ray sensor is generally formed by a pyroelectric member provided with electrodes on the front and back surfaces thereof and is excellent in sensitivity to slight temperature difference, whereas the same is liable to be affected by thermal noise and may be driven by a heat source such as a light of an automobile or an abrupt change in ambient temperature.
In order to prevent erroneous operation through such noise components other than the intruder, the so-called dual sensor device has been proposed and put into practice, in which oppositely polarized two pyroelectric infrared ray detecting elements are coupled in series or parallel with each other.
U.S. Pat. No. 3,839,640 discloses an example of such a dual sensor device.
This dual sensor device utilizes output of difference with signals obtained from two elements so that the two elements cancel influence through a temperature change simultaneously applied thereto or a change in ambient temperature, whereby no erroneous operation is caused by such external noise and the intruder can be stably detected.
FIG. 1 shows such a conventional dual sensor device. Referring to FIG. 1, first and second infrared ray detecting elements 1 and 2 are provided in a parallel manner in the vicinity of the focus F of a parabolic mirror 3. When a detected object moves from a point .alpha. to a point .beta. and then from the point .beta. to a point .gamma. at uniform velocity, the first infrared ray detecting element 1 generates an output signal a as shown in FIG. 2A during the movement from the point .alpha. to the point .beta., and the second infrared detecting element 2 generates an output signal b during the movement from the point .beta. to the point .gamma.. The output of the difference between these two signals a and b is as shown in FIG. 2B, from which it is obvious that a large signal level can be obtained.
However, the output signal waveforms of FIGS. 2A and 2B merely show a case where only straight heat rays enter only a light receiving surface of the dual sensor device. However, in practice heat rays generally enter the device from every direction, and thus actual output signals are substantially as shown in FIG. 3A. FIG. 3A shows output signals a and b actually obtained from the infrared ray detecting elements 1 and 2 of the dual sensor device, and the difference between the output thereof is extremely lower in output level at its center c, as shown in FIG. 3B.
In a general infrared ray detector, a pyroelectric infrared ray sensor is inferior in input sensitivity and hence a large-dimensional concave parabolic mirror is employed. The infrared ray sensor is fixed in the vicinity of its focus thereby to improve the signal-to-noise ratio by increasing the amount of heat rays entering the infrared ray sensor. Thus, an infrared ray detector having an excellent signal-to-noise ratio is inevitably increased in size.
Another type of an infrared ray detector is provided with dividing segment spherical mirror means prepared by dividing a parabolic mirror into a plurality of sections, in order to detect objects such as intruders approaching from various directions. Also in this case, the divided mirror sections themselves are increased in size in order to retain output from the infrared ray sensor in an excellent signal-to-noise ratio, and hence the entire infrared ray detector is increased in size to remarkably restrict the position of installation.
In addition, an infrared ray detector is mainly directed to detect the intruder. The output from the infrared ray sensor following the intruder movement signal is in a frequency range of about 0.1 to 10 Hz. In a circuit for processing signals with such a low frequency range, the capacitor of a filter circuit is indispensably increased in capacitance which requires a large space, and hence it has been difficult to reduce the size of the infrared ray detector.